1. Field of the Invention
This invention relates to the thermal processing of glass sheets, and particularly to the thermal processing of glass sheets suspended by metal tongs of the self-closing type having a greater heat capacity per unit area than said glass sheets undergoing thermal processing. During thermal processing required to heat strengthen, temper, anneal, shape, and/or coat the glass sheet, it is necessary to heat the glass to above its strain point and then cool the glass sheet. Intermediate processing steps, such as shaping and/or coating, may also be performed between the heating and the cooling steps.
When glass sheets are supported in a vertical position for such thermal processing by suspension from metal tongs having a greater heat capacity per unit area than that of the glass, conventional prior art heating elements within a furnace do not heat the tongs sufficiently to enable the glass sheet in the vicinity of the tongs to develop the high temperature needed for subsequent treatment. Since the upper edge portion of the glass sheet near each vicinity gripped by tongs tends to be cooler than the remainder of the glass sheet, the glass sheet frequently develops vents in the vicinity of the region where the tongs grip the glass. Such vents cause rejects and even cause the glass to break spontaneously either during or after processing.
Attempts were made to reduce the frequency of tong vents by preheating the tongs before the tongs engaged the upper edge of glass sheets to be thermally processed. However, there are several drawbacks involved in preheating tongs in such a manner. First of all, it is difficult for operators loading glass sheets into tongs to work with hot tongs. They must use heat-insulating gloves, whose thickness makes it difficult to manipulate the tongs and/or the glass sheets undergoing processing. Furthermore, preheating the tongs prior to engaging the glass sheet is uneconomical because it wastes much heat and is imprecise because it is difficult to control the amount the tongs cool during the time interval between the preheating and the tong loading steps.
Also, the glass in the vicinity of preheated tongs may become heated to a higher temperature than the rest of the glass during the early stages of the heating cycle for the glass sheets. This initial heating may develop local distortion of the glass in the vicinity of the tongs. Such a phenomenon is called "tong pull". The amount of time as well as the temperature of the glass sheet portion in the vicinity of the tongs at the elevated temperature determines the severity of "tong pull". The localized areas of distortion that result from "tong pull" are not only out of tolerance with respect to the shape desired for the glass, but also result in optical defects or tong marks that result when the tongs penetrate deeply into the heat-softened glass. Such optical defects must be avoided in dealing with glass sheets that are fabricated into viewing windows, such as tempered side lights and back lights of automobiles, or tempered coated sheets used as windows in buildings and elsewhere.
In summation, the difficulty of using tongs to grip glass sheets for thermal processing prior to the present invention involved temperature gradients between the glass sheet portions in the vicinity of the tongs and the remainder of the glass sheets. If the tongs were too cold relative to the glass, tong vents occurred in the glass. If the tongs were too hot relative to the glass, "tong pull" and deep tong marks in the glass from deep tong penetration into the softened glass resulted.
2. Description of the Prior Art
The following patents disclose the general development of furnaces used to heat glass sheets while supported by tongs during heating.
U.S. Pat. No. 3,293,020 to Sleighter discloses a furnace for processing tong-gripped glass sheets. The furnace is provided with a series of electrical heaters 42 supported along the inner side walls 44 and 46 thereof. The voltage to each electrical heater may be controlled. However, Sleighter provides no indication as to how the voltage for individual heaters may be controlled so as to heat glass gripping tongs independently of the glass sheets.
U.S. Pat. No. 3,295,843 to McMaster discloses an enclosed furnace for heating an enlarged tong-gripped glass sheet wherein the radiating temperature of the heating elements is controlled to provide uniform temperature throughout the entire extent of the glass sheet. This patent concerns the heating of glass sheets, not the heating of tongs.
U.S. Pat. No. 3,744,985 to Peternel discloses a furnace through which glass sheets are conveyed while suspended from tongs. The furnace is sub-divided into six zones in the horizontal direction, with each zone comprising three sections arranged vertically, one above another. The input to heating elements is controlled in each of the sections of each of the zones. The heating furnace in this patent is used in conjunction with a glass shaping apparatus where the glass is press bent to shape after it leaves the furnace and means is provided to control the thermal input to preselected control zones in response to local deviations from desired shape of the press bent glass sheets. The remaining zones are programmed to be adjusted automatically in response to variations in temperature of different glass sheet regions compared to set point temperatures established for securing proper localized curvature for corresponding regions of the bent glass. No provision is made in this furnace for selectively controlling the intensity of heat supplied to heaters facing the tongs as distinguished from heaters facing the glass sheets passing through the furnace en route to the shaping station.
U.S. Pat. No. 3,854,920 to Kay et al. discloses a furnace for processing glass sheets suspended from tongs and having heating elements 23 mounted on side walls, heating elements 24 mounted on the floor and heating elements 25 mounted on the roof. Certain of these heating elements face the tongs 8 and other heating elements face the glass sheets 7. However, there is no teaching in this patent of separately controlling the heat to be applied to the tongs so as to have it different from the intensity of heat applied to the glass sheet regions remote from the tongs.
To the best of our knowledge, the prior art did not provide in a furnace for heating tong-gripped glass sheets means for independently supplying energy to heaters facing the path taken by the tongs independently of the means for providing energy to heaters facing the path taken by a glass sheet gripped by the tongs to provide simultaneously different increments of heat to the tongs and to the glass sheet during their passage through the furnace during the heating phase of thermal processing.